1. Field of the Invention
The present invention relates to photolithography techniques used in semiconductor device manufacturing processes. Specifically, the present invention relates to a multi-layer, attenuated phase-shifting mask or reticle that reduces problems associated with side lobe printing in areas including closely spaced or nested features, while maximizing resolution and depth-of-focus performance for isolated features of a semiconductor device.
2. State of the Art
Photolithography processes are essential to the fabrication of state of the art semiconductor dice. Such processes are used to define various semiconductor die features included in semiconductor dice and generally include exposing regions of a resist layer to patterned radiation corresponding to the semiconductor die circuit feature to be defined in a substrate underlying the layer of resist. After exposure, the resist layer is developed to selectively reveal areas of the substrate that will be etched to define the various device features while selectively protecting those areas of the substrate which are not to be exposed to the etching process. In order to properly form a radiation pattern over a resist layer, the radiation is generally passed through a reticle or mask which projects the semiconductor die feature pattern to be formed in the resist layer.
Various types of photolithographic masks are known in the art. For example, known masks often include a transparent plate covered with regions of a radiation blocking material, such as chromium, which define the semiconductor die feature pattern projected by the mask. Such masks are called binary masks since radiation is completely blocked by the radiation blocking material and fully transmitted through the transparent plate in areas not covered by the radiation blocking material. However, binary masks cause significant fabrication problems, particularly where semiconductor die dimensions shrink below 1 μm.
As the pattern features of a binary mask are defined by boundaries between opaque, radiation blocking material and material which is completely radiation transmissive, radiation passing through a binary mask at the edge of a pattern feature will be diffracted beyond the intended image boundary and into the intended dark regions. Such diffracted radiation prevents formation of a precise image at the feature edge, resulting in semiconductor die features which deviate in shape or size from the intended design. Because the intensity of the diffracted radiation drops off quickly over a fraction of a micron, diffraction effects are not particularly problematic where semiconductor dice have dimensions on the order of 1 μm. However, as feature dimensions of state of the art semiconductor dice shrink well below 0.5 μm, the diffraction effects of binary masks become terribly problematic.
Another type of mask known in the art is an attenuated phase shift mask (APSM). APSMs were developed to address the diffraction problems produced by binary masks and are distinguished from binary masks in that, instead of completely blocking the passage of radiation, the less transmissive regions of the mask are actually partially transmissive. Importantly, the attenuated radiation passing through the partially transmissive regions of an APSM generally lacks the energy to substantially affect a resist layer exposed by the mask. Moreover, the partially transmissive regions of APSMs are designed to shift the passing radiation 180° relative to the radiation passing through the completely transmissive regions and, as a consequence, the radiation passing through the partially transmissive regions destructively interferes with radiation diffracting out from the edges of the completely transmissive regions. Thus, the phase shift greatly reduces the detrimental effects of diffraction at the feature edges, thereby increasing the resolution with which sub-micron features may be patterned on a resist layer.
A conventional APSM 4 is illustrated in drawing FIG. 1. As can be seen, the APSM 4 includes a transparent substrate 6 coated with a partially transmissive material 7 (to ease description, drawing FIG. 1 provides a greatly simplified APSM). The partially transmissive material 7 has been patterned to form a completely transmissive region 8 and two attenuated regions 10a, 10b. The attenuated regions 10a, 10b of a typical APSM 4 are typically designed to allow the passage of between about 4% (low transmission) and 20% (high transmission) of the incident radiation 12. The partially transmissive material 7 forming the attenuated regions 10a, 10b is formed to a thickness that shifts the incident radiation 12 one hundred eighty degrees (180°) out of phase.
Also provided in drawing FIG. 1 is a graph 16 illustrating the electromagnetic intensity (plotted on the vertical axis) of the radiation passing through the APSM 4 relative to the position (plotted on the horizontal axis) on the surface of the exposed resist. As shown, the intensity curve 18 includes a first component 20 located primarily between the edges 22a, 22b formed between the attenuated regions 10a, 10b and the completely transmissive region 8 of the APSM 4. The first component 20 of the intensity curve 18 corresponds to the electromagnetic intensity of the radiation passing through the completely transmissive region 8 of the APSM 4 illustrated in drawing FIG. 1. As can be seen in the graph 16, the electromagnetic intensity of the radiation falls to zero at points 24a, 24b, which are near the edges 22a, 22b. Points 24a, 24b correspond to the locations where the magnitudes of the in phase radiation passing through the completely transmissive region 8 and the out of phase radiation passing through the attenuated regions 10a, 10b are equal. Beyond points 24a, 24b and moving away from the edges 22a, 22b, the electromagnetic intensity of the transmitted radiation grows again to a steady value as indicated by the second curve components 26a, 26b. The second curve components 26a, 26b represent the electromagnetic intensity of the radiation passing through the attenuated regions 10a, 10b of the APSM 4.
The electromagnetic intensity represented by the second curve components 26a, 26b is also known as “ringing effects,” and one significant disadvantage of APSMs is that such ringing effects become much more severe as feature density of an APSM increases. As device features designed into an APSM are spaced closer and closer together, the ringing effects of adjacent device features begin to overlap, and as the ringing effects overlap, the electromagnetic intensity of such ringing effects becomes additive. These increased ringing effects are known as “additive side lobes,” “additive ringing effects,” or “proximity effects.” In contrast to isolated ringing effects produced by isolated device features, the electromagnetic intensity of additive side lobes created by closely spaced (i.e., ≦0.5 μm) or nested device features often becomes sufficiently intense to cause printing of the resist layer, which is commonly termed “side lobe printing.”
Illustrated in drawing FIG. 2 is the additive ringing effects associated with conventional APSMs having closely spaced feature formations. As illustrated in drawing FIG. 2, a second APSM 30 includes a transparent substrate 32 coated with a partially transmissive phase-shifting film 34 (again, for ease of description, drawing FIG. 2 provides a greatly simplified APSM). The partially transmissive phase-shifting film 34 has been patterned to form four attenuating regions 36a-36d and three completely transmissive regions 38a-38c, which are closely spaced. Radiation 39 incident on the APSM 30 passes through the completely transmissive regions 38a-38c and the attenuated regions 36a-36d and impinges upon the surface of the resist layer to be patterned (not illustrated in drawing FIG. 2).
Included in drawing FIG. 2 is a graph 40 illustrating the electromagnetic intensity of the radiation incident upon the surface of the resist layer to be patterned. The graph 40 includes an intensity curve 42 made up of various components, with the first components 43 a-43 c illustrating the electromagnetic intensity of the radiation passing through the completely transmissive regions 38a-38c of the APSM 30, the second components 44a, 44b illustrating the electromagnetic intensity of the ringing effects produced by the radiation passing through the isolated attenuated regions 36a, 36d, and the third components 46a, 46b illustrating the electromagnetic intensity of the additive side lobes produced by the dense feature arrangement formed by the closely spaced attenuated regions 36b, 36c. As can be seen in drawing FIG. 2, the magnitude of the second components 44a, 44b (represented by line “I1”), which illustrate the intensity of the ringing effects produced by isolated attenuated regions 36a, 36d, is significantly lower than that of the third components 46a, 46b (represented by line “I2”), which illustrate the electromagnetic intensity of the additive side lobes.
Provided in drawing FIG. 3 is a cross-sectional view of a partially fabricated structure 50 after exposure through the APSM 30 illustrated in drawing FIG. 2. The partially fabricated structure 50 includes a semiconductor substrate 52 and a developed resist layer 54. The developed resist layer 54 exhibits a set of depressions 56a, 56b resulting from the relatively high electromagnetic intensity of the additive side lobes caused by the dense feature arrangement of the APSM 30. As device feature density increases, so will the intensity of the additive side lobes and the extent to which the resist layer is patterned due to exposure to additive ringing effects. Thus, depressions in the resist layer due to additive ringing effects may, in some situations, degrade the resist layer to such an extent that entire semiconductor dice become unusable due to damage incurred during a subsequent etch process.
As is well known in the art, the ringing intensity is inversely related to the attenuation of the partially transmissive material used in APSMs. Increasing the attenuation of the partially transmissive material will, therefore, decrease any resultant ringing effects, while decreasing the attenuation will increase any resultant ringing effects. Thus, the intensity of additive side lobes produced by closely formed features in an APSM may be decreased simply by increasing the attenuation of the partially transmissive regions included therein.
However, increasing the attenuation of the partially transmissive areas of APSMs also has significant drawbacks. For example, increasing the attenuation of the partially transmissive areas decreases print performance as well as the resolution and depth-of-focus achievable by the APSM. Reduction of depth-of-focus and resolution characteristics of an APSM are particularly problematic in the fabrication of state of the art semiconductor devices, which requires that an APSM accurately project images corresponding to sub-0.5 μm device features while focusing such images through relatively thick layers of resist. In addition, even with the most precise fabrication equipment, sub-micron deviations from the optimum focus position of the APSM relative to the resist layer to be patterned will occur, and decreasing the depth-of-focus of an APSM increases the probability that fabrication defects may result from such slight deviations. Therefore, increasing the attenuation of the partially transmissive materials included in state of the art APSMs requires a careful compromise between control of additive ringing effects and maximization of resolution and depth-of-focus performance.
Furthermore, state of the art semiconductor dice often include closely spaced or nested features as well as features which are isolated. It would, therefore, be an improvement in the art to provide an APSM that includes highly attenuated regions (i.e., attenuating regions allowing about 4% to about 10% transmittance of incident radiation) where necessary to control additive ringing but also includes slightly attenuated regions (i.e., attenuating regions allowing about 12% to about 20% transmittance of incident radiation) where isolated device features are to be formed. Such an APSM would enable control of additive ringing effects where needed without compromising resolution and depth-of-focus performance where additive ringing effects are of little or no concern.